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IN A PERSPECTIVES ARTICLE “Learning from monkey ‘talk’ ” (17 March, p. 1120), Charles Snowdon suggests that monkeys – macaques and baboons – have “vocal tracts capable of human-like speech,” and wonders “why non-human primates don’t talk.” But the presupposition that the ability to talk leads to full language is false.
Human language is neither speech (because internal language can be spoken, signed, or haptic) nor communication (though externalized language can be used this way). Furthermore, speech and sign display similar developmental trajectories, subserved by the same neural substrate (1). It is a further mystery then why non-human primates did not simply develop sign from gesture. Human infants, in contrast, spontaneously develop sign language without linguistic input. Deaf-blind children (famously, Helen Keller) develop internal language in the absence of speech or sign. Speech and sign are thus simply input-output channels clearly not necessary, and obviously not sufficient for language. Rather what matters is language’s internal computational “operating system.”
The basic property of this “operating system” generates an unbounded array of hierarchically structured expressions with systematic meanings, antedating speech or sign, virtually as a matter of logic (2-4). It relies only on structural properties, ignoring linear properties of the externalized signal (5, 6). Though the...

IN A PERSPECTIVES ARTICLE “Learning from monkey ‘talk’ ” (17 March, p. 1120), Charles Snowdon suggests that monkeys – macaques and baboons – have “vocal tracts capable of human-like speech,” and wonders “why non-human primates don’t talk.” But the presupposition that the ability to talk leads to full language is false.
Human language is neither speech (because internal language can be spoken, signed, or haptic) nor communication (though externalized language can be used this way). Furthermore, speech and sign display similar developmental trajectories, subserved by the same neural substrate (1). It is a further mystery then why non-human primates did not simply develop sign from gesture. Human infants, in contrast, spontaneously develop sign language without linguistic input. Deaf-blind children (famously, Helen Keller) develop internal language in the absence of speech or sign. Speech and sign are thus simply input-output channels clearly not necessary, and obviously not sufficient for language. Rather what matters is language’s internal computational “operating system.”
The basic property of this “operating system” generates an unbounded array of hierarchically structured expressions with systematic meanings, antedating speech or sign, virtually as a matter of logic (2-4). It relies only on structural properties, ignoring linear properties of the externalized signal (5, 6). Though there is apparent convergent evolution for auditory-vocal imitation learning in humans and song-learning birds (7), there is no evidence for any kind of animal equivalent to human recursive syntax (8-10).
The article advocates a suite of nonhuman models for the study of evolution of language and speech. But the human capacity for language is not the sum of the vocalizing capacity of songbirds and referential signaling of monkeys. To be sure, humans can signal referentially and humans can vocalize, but, crucially, only humans have the syntax and concepts that are otherwise absent in the animal world (4, 8).